This invention relates to the temperature control of oxygen concentration sensors, and more particularly to a temperature control device for an oxygen concentration sensor of the type which generates an output proportional to the concentration of oxygen in a gaseous substance such as exhaust gases emitted from an internal combustion engine.
An air-fuel ratio control system for an internal combustion engine is generally employed, which senses the concentration of oxygen in exhaust gases emitted from the engine by means of an oxygen concentration sensor. The control system controls the air-fuel ratio of a mixture supplied to the engine, in a feedback manner responsive to the output signal from the oxygen concentration sensor, to thereby purify the exhaust gases and improve the fuel consumption, etc.
In recent years, a type of oxygen concentration sensor has been developed for use in the air-fuel ratio control system, which generates an output proportional to the concentration of oxygen contained in the exhaust gases. A sensor of this type is disclosed, e.g. in Japanese Provisional Patent Publication (Kokai) No. 59-192955, which comprises an oxygen-pumping element and a cell element, each being composed of a plate-like member formed of a solid electrolytic material having oxygen ion-conductivity, and a couple of electrodes. A gas-staying chamber is provided as a gas diffusion-limiting zone between the oxygen-pumping element and the cell element, with one of the coupled electrodes of each element being attached to one of opposite inner walls of the chamber. A gas to be examined is introduced into the gas-staying chamber through a gas-introducing slit. An air chamber into which the atmosphere is introduced is provided adjacent the cell element, with the other of the coupled electrodes of the cell element facing the interior of the air chamber.
According to the disclosed sensor, when a voltage developed between the two electrodes of the cell element is above a predetermined reference voltage, a voltage corresponding to the difference between them is supplied to the oxygen-pumping element so that oxygen ions move through the oxygen-pumping element toward the electrode located in the gas-staying chamber or gas diffusion-limiting zone. When the voltage between the two electrodes of the cell element is below the predetermined reference value, a voltage corresponding to the difference between them is supplied to the oxygen-pumping element so that oxygen ions move toward the electrode located outside the gas-staying chamber. Thus, electric current flowing between the two electrodes of the oxygen-pumping element, i.e. pumping current, assumes a value proportional to the concentration of oxygen in the gas supplied into the gas-staying chamber or gas diffusion-limiting zone.
If an oxygen concentration sensor of the above described type which provides an output proportional to the oxygen concentration is used in feedback control of the air-fuel ratio in an internal combustion engine, attain an output characteristic proportional to the oxygen concentration, the solid electrolytic members have to be maintained at a temperature e.g. 650.degree. C. or more. This temperature is considerably higher than the temperature of exhaust gases, so as to impart sufficient oxygen ion-conductivity to the elements. To this end, an electrical heating element is arranged for the solid electrolytic members at a suitable portion thereof, for heating the same. The heating element is supplied with heating current from a temperature control device. Only after the solid electrolytic members have been fully heated is the air-fuel ratio feedback control responsive to the sensor output started.
As the above temperature control device, a device is disclosed, e.g. in Japanese Provisional Patent Publication (Kokai) No. 60-48518, which comprises a series circuit composed of an electrical heating element and a switching element which are serially connected to each other. A predetermined direct current voltage is applied to the series circuit and a heating current flowing in the heating element is detected. The switching element is turned on and off by driving pulses with a predetermined pulse repetition period at a duty factor corresponding to the magnitude of the heating current. Thus, the heating current applied to the heating element is controlled to desired values by means of the duty factor control.
In the case where the oxygen concentration sensor is used to detect oxygen concentration in exhaust gases from an internal combustion engine, there is a possibility of overheating the sensor when the engine is operating in a high load condition. If the sensor is kept in such an overheated state for a long time, it can deteriorate. One possible way to solve the problem is to detect the temperature of the heating element from the resistance value of same, and interrupt the supply of heating current to the heating element immediately when the resistance value exceeds a predetermined critical value so as to allow the heating element to be cooled. The resistance value may be detected from the value of heating current or the voltage across the heating element. However, in the case where the heating current is controlled by means of the duty factor control as stated above, it is impossible to detect the resistance value during the off period. Further, the pulse repetition period is very short, e.g. 100 ms, which is inappropriate for quick detection of the resistance value.